1. Field of the Invention
This disclosure relates to preparation of rare earth materials, such as the preparation of phase-pure garnet particles.
2. Description of the Related Art
Direct production of garnets such as yttrium aluminum garnet (YAG) is possible via several different wet chemistry as well as solid state and combustion routes. For solid state or combustion synthesis, improving particle size control is a current research goal. On the other hand, a current goal in the area of wet chemical processes is reduction of organic contamination and defective crystalline structure. Improving any of these may improve the desirable properties of garnets for various applications viz. optical properties when used as phosphor hosts, lasing materials or scintillators.
Flow-based thermochemical syntheses techniques like thermal plasma based methods, flame spray pyrolysis, spray pyrolysis, and other processes of a similar nature are promising because they may reduce contaminants and improve control over particle shapes and sizes. These processes are also very suitable for continuous production compared to the batch processing nature of wet synthesis.
However, there is a need for a better method to create phase-pure polycrystalline garnet phase materials, using flow-based thermochemical synthesis methods. The current flow-based thermochemical methods tend to produce powders that are not phase-pure. The synthesized powders may contain garnets and other thermodynamic phases such as amorphous or different crystalline phases. These powders which are not phase-pure garnet material may not possess the desired characteristics that make garnet materials useful in many applications.
For example, there are several problems associated with the processing of yttrium aluminum garnet (YAG). The yttria-alumina (Y2O3—Al2—O3) system exhibits several other stable phases, including Y2O3 (yttria), monoclinic Y4A12O9 (YAM [yttrium aluminum monoclinic]), hexagonal or orthorhombic YAlO3 (perovskite or YAP [yttrium aluminum perovskite]) and Al2O3 (alumina). These alternative thermodynamic phases do not share the same optical characteristics as YAG. Unfortunately, the current state-of-the-art process steps useful in the manufacture of YAG ceramics or YAG powders may lead to the formation of these undesired phases. Powders or films composed of these alternative thermodynamic phases may be converted to the desired garnet phase upon application of suitable additional heat treatment/annealing at high temperatures, but this may dramatically increase total processing time and cost. Heating or annealing may also result in sintering of the particles which may cause loss of control of particle size.
Thus there is a need for a suitable process for generating phase-pure garnet materials using processes suitable for continuous production without resorting to subsequent thermal annealing steps. There also may be a need to improve control of particle size.